A load-sensitive automatic transmission system for an agricultural electric vehicle is provided, which includes a drive motor for generating a rotational power using electric power of a battery; a transmission for changing a rotational speed of the drive motor in multiple stages and outputting the changed rotational speed; a forward/reverse differential gear for transmitting the power of the transmission to wheels; a control unit for detecting a running state of the vehicle and a load state of the drive motor and determining a speed change time; and an actuator for operating the transmission in response to a signal from the control unit. Accordingly, it is possible to selectively provide high speed and torque to meet the needs of different situation, beyond the limitations of the motor drive performance of a conventional electric vehicle.